1. Field of the Invention
The present invention relates to a motor control apparatus equipped with a delta-sigma modulation AD converter, and more specifically to a motor control apparatus equipped with a delta-sigma modulation AD converter that is used to detect current flowing in each winding of a motor.
2. Description of the Related Art
A motor control apparatus for driving motors used in a machine tool, a forging press, an injection molding machine, an industrial robot, or the like, commands motor speed, torque, or rotor position in order to control the operation of each of the motors provided one for each drive axis. In such a motor control apparatus, the current flowing in each winding of the motor is detected and AD (analog-digital) converted into digital data which is used to control the driving of the motor as well as to detect any abnormal current flow that may occur in the motor.
FIG. 3 is a block diagram showing a conventional motor control apparatus. Generally, the motor control apparatus 101 which drives and controls a three-phase AC motor 2 includes a power conversion unit 51 which supplies drive power to the motor 2, a current detection unit 52 which detects the value of the current flowing from the power conversion unit 51 to each winding of the motor 2, a delta-sigma modulation AD (analog-digital) converter 53 which converts the value of the current detected by the current detection unit 52 into digital data, and a command generating unit 54 which generates, using the digital data supplied from the delta-sigma modulation AD converter 53, a drive command for commanding the power conversion unit 51 to output the drive power for driving the motor 2. The power conversion unit 51 is, for example, an inverter circuit and/or a converter circuit. The current detection unit 52 actually comprises two current detection units one for each of two of the three phase windings of the three-phase AC motor. In the thus configured motor control apparatus 101, the command generating unit 54 generates the drive command based on the digital data acquired by AD-converting the current flowing in each winding of the motor 2. Further, in the motor control apparatus 101, the digital data acquired by AD-converting the current flowing in each winding of the motor 2 is also used to monitor any abnormal current flow that may occur in each winding of the motor 2.
In recent years, the delta-sigma modulation AD conversion scheme described above has come to be widely employed in motor control apparatus. FIG. 4 is a block diagram showing a conventional delta-sigma modulation AD converter. The delta-sigma modulation AD converter 53 comprises two major sections, i.e., a modulator (delta-sigma modulation circuit) 61 and a digital low-pass filter 62, both of which operate on a system clock called a modulation clock whose frequency is generally about several to several tens of megahertz. The modulator 61 converts the input analog data into a high-speed low-bit bitstream signal. A large amount of quantization noise generated is removed by the digital low-pass filter 62, and the resulting data is output as the digital data. Generally, the digital data from the delta-sigma modulation AD converter 53 is output at a rate reduced by thinning the modulation clock by a predetermined factor. This factor is generally called the decimation ratio.
The delta-sigma modulation AD converter 53 in the motor control apparatus 101 is required to convert the value of the drive current flowing in each winding of the motor 2 into digital data with high resolution and high accuracy in order to control the speed, torque, or rotor position of the motor 2 with high accuracy. In the motor control apparatus 101 shown in FIG. 3, the command generating unit 54 acquires at a constant control rate the digital data that the delta-sigma modulation AD converter 53 outputs by AD-converting the value of the current flowing in each winding of the motor 2, and uses the acquired digital data to control the current. To acquire the digital data with high resolution and high accuracy, the sampling period of the delta-sigma modulation AD converter 53 is generally set to several tens to several hundreds of microseconds.
On the other hand, the motor control apparatus 101 is equipped with a protection circuit for detecting an abnormal current flow when an abnormal excessive current flows in any one of the windings of the motor 2, and for protecting the motor 2 and the motor control apparatus 101 against the abnormal current. Since such an abnormal current needs to be detected quickly, it is necessary for the delta-sigma modulation AD converter 53 in the motor control apparatus 101 to convert the value of the drive current flowing in each winding of the motor 2 into digital data as fast as possible. For example, in order to quickly detect abnormal current flow, it is generally necessary that the sampling period of the delta-sigma modulation AD converter 53 be several microseconds or shorter, but it is not necessarily required that the digital data obtained as the result of the AD conversion be high-resolution and high-accuracy data.
For a motor control apparatus that controls a motor by using a delta-sigma modulation AD converter for converting the current value detected on each winding of the motor into digital data, several proposals have been made in the prior art to enhance the current detection accuracy, including the one disclosed in Japanese Unexamined Patent Publication No. 2008-147809. According to the invention disclosed in Japanese Unexamined Patent Publication No. 2008-147809, the current detection accuracy is enhanced by additionally providing a PLL circuit in order to enhance the accuracy of the modulation clock in the modulator section provided in the first stage of the delta-sigma modulation AD converter.
On the other hand, according to the invention disclosed in Japanese Patent No. 3348036, a delta-sigma modulation DA (digital-analog) converter that can accomplish high-fidelity power amplification has been proposed.
As described above, in the motor control apparatus, the processing speed (response speed) and resolution required of the delta-sigma modulation AD converter for AD-converting the value of the current flowing in each winding of the motor differs between the motor control function and the motor protection function. To address this situation, the prior art employs a method that provides a plurality of delta-sigma modulation AD converters one for each specific function, or a method that uses the digital data output from a single delta-sigma modulation AD converter for both the motor control function and the motor protection function. However, the method that uses a plurality of delta-sigma modulation AD converters has had the problem that the mounting area in the motor control apparatus and the cost of the apparatus increase, while the method that uses the digital data output of a single delta-sigma modulation AD converter for both functions has the problem that the motor drive control accuracy and the abnormal current detection accuracy both degrade, because the processing speed (response speed) and resolution of the AD conversion are not optimized for each specific function.
For example, the invention disclosed in Japanese Unexamined Patent Publication No. 2008-147809 merely provides a single digital data output by using a single delta-sigma modulation AD converter, and it cannot be said that the processing speed (response speed) and resolution of the AD conversion are optimized for each specific function; hence, this prior art has involved the problem that the motor drive control accuracy and the abnormal current detection accuracy both degrade.
On the other hand, the invention disclosed in Japanese Patent No. 3348036 is intended for audio equipment and is not applicable to motor control apparatus. Furthermore, since this prior art is intended for a DC converter for converting digital data into an analog signal, it does not provide any solution when converting the current value flowing in a motor into digital data.